Polarity reversal system for electrical discharge machining apparatus

ABSTRACT

A control system is provided with an electronic switch control network for maintaining a constant polarity gap output signal from a single or multiple electrode gap system independently of whether the electrical power conducting leads from the power source are connected to one or the other of the EDM gap elements. The gap output signal which is ordinarily representative of gap voltage is then utilized in other associated EDM circuits such as for an example power feed control networks and machining pulse cut-off control networks to regulate their operation.

United States Patent [191 Verner 1 May 8,1973

[54] POLARITY REVERSAL SYSTEM FOR ELECTRICAL DISCHARGE MACHINING APPARATUS [75] Inventor: Dalton R. Verner, Orchard Lake,

Mich.

[73] Assignee: Elox lnc., Davidson, NC.

[22] Filed: Mar. 10, 1971 [21] Appl. N0.: 122,830

[52] U.S. Cl. ..219/69 C, 219/69 G, 219/69 8 [51] Int. Cl. ..B23p 1/08, B23p 1/14 [58] Field of Search ..2l9/69 C, 69 G, 69 P,

219/69 R, 69 S, 69 V [56] References Cited UNITED STATES PATENTS 3,590,205 6/197] Verner et al ..2l9/69 G 2,273,819 2/1942 Cooke et al. ..2l9/69 V 8/1960 Ullmann ..2l9/69 C 3,492,593 l/l970 Ullmann et al. ..2l9/69 P X 3,624,338 ll/l97l Ellis et al. ..2l9/69 C Primary Examiner--R. F. Staubly Attorney-Haukc, Gifford & Patalidis [57] ABSTRACT A control system is provided with an electronic switch control network for maintaininga constant polarity gap output signal from a single or multiple electrode gap system independently of whether the electrical power conducting leads from the power source are connected to one or the other of the EDM gap ele-' ments. The gap output signal which is ordinarily representative of gap voltage is then utilized in other associated EDM circuits such as for an example power feed control networks and machining pulse cut-off control networks to regulate their operation.

19 Claims, 2 Drawing Figures DR VE PATENTED MAY 8 75 SHEET 1 [IF 2 Emu WA QLU UW Y UQ N flaw mm DALTON SHEET 2 OF 2 PATENTED Y 8191a N TX "(a m f Q M Am 3" TWT BY fi wl r m I ATTORNEYS POLARITY REVE RSAL SYSTEM FOR ELECTRICAL DISCHARGE MACHINING APPARATUS BACKGROUND OF THE INVENTION The field to which my invention relates is that known generally as electrical discharge machining, hereinafter sometimes referred to as EDM. In this process, material is removed from an electrically conductive workpiece by electrical gap discharges which are caused to occur between a tool electrode and the workpiece. A servo feed system for the electrode or workpiece is used to provide relative movement and thus maintain an optimum gap spacing between the electrode and workpiece as the material is removed. A dielectric coolant liquid is circulated and recirculated, usually under pressure, through the gap during machining operation. For most reliable and predictable results, a power supply circuit of the independent pulse generator type is utilized to provide machining pulses of precisely controllable frequency and on-off time characteristic. During the machining operation, the gap may become bridged by workpiece or electrode particles to cause a condition commonly known as gap short circuit condition. This condition is accompanied by excessive localized heat which tends to damage both electrode and workpiece unless prompt corrective action is taken. Corrective action is provided by systems which have been devised to provide fast acting servo withdrawal or, alternately or concommitantly, interruption of machining current to the gap. A basic requirement of either system is that a reliable control output signal be received, normally from the machining gap itself, which signal indicates the level of gap voltage and the changes which are occurring or impending. Gap short circuit condition is normally accompanied by an abrupt drop of gap voltage. This change in gap voltage signal is usually monitored by sensing leads which are connected to the two gap elements. As different tool electrode and workpiece material combinations are used, it sometimes becomes necessary to change the polarity from the standard condition, that is, with electrode negative and workpiece positive, to the opposite or reverse polarity condition. This, of course, causes a problem with respect to the sensing leads and to the other internal control circuits for servo control or current cut-off control. My polarity reversal system is able to overcome these problems and to provide a ready means for changeover of gap polarity in one machining gap or in a plurality of machining gaps. The polarity of gap output signal is maintained constant for controlling the various functions of the associated circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS The following is a brief description of the drawings in which like numerals and letters are used to refer to like elements where they occur throughout the several views;

FIG. 1 is a combined schematic and block diagrammatic showing of an electrical discharge machining power supply with a polarity reversal system as used in conjunction with a servo control circuit; and

FIG. 2 is a drawing substantially similar to FIG. I in which the polarity reversal system is illustrated in conjunction witha machining current cut-off circuit.

DESCRIPTION Referring now to FIG. 1 of the drawings, an EDM system is shown which includes multiple machining gaps. A main machining power source 10 for the left hand machining gap is shown connected in series with a pair of output transistors 12 and 12' and in series with that machining gap comprising a tool electrode 14 and a workpiece 16. The magnitude of gap cutting current is controlled by the setting of a pair of resistors l8, 18. A rectifier 20 is connected in series between electrode 1 l4 and the positive terminal of the power source 10.

A second machining gap is shown at the right hand side of the drawing, which gap similarly includes a main machining power source 22 and a pair of output stage transistors 24, 24 connected with that machining gap comprising a tool electrode 26 and a workpiece 28. Gap current magnitude is controlled by the setting of a pair of resistors 30, 30'. A rectifier 24 is shown connected in series between the electrode 26 and the positive terminal of the power source 22. It will be seen that the EDM power supply circuit illustrated is suitable for operation in either of several difierent modes. For one type of operation, it is possible to combine the two output circuits to machine simultaneously two or more holes in a common workpiece, in which case workpieces 16 and 28 would comprise the same electrically conductive body. Alternately, for another common type of operation, it would be possible to maintain simultaneous cutting operation on two separate and spaced workpieces 16 and 28 in the manner illustrated. The present invention is of particular importance with respect to that system in which a plurality of electrodes are mounted on a common machine tool ram, controlled by a single servo feed system and used to machine a number of holes in the same workpiece or in a number of workpieces at the same time.

Included in circuit with the two output stages shown are corresponding intermediate drive stages 30, 30a, and 32, 32a, respectively. In connection with the left hand machining gap, a final drive stage transistor 33 is connected in the manner shown to operate the output transistors 12, 12'. A final drive stage transistor 34 is connected to drive the output stage for the right hand machining gap including the output transistors 24 and 24'. Resistors 36, 38 and 39 are connected across the respective base emitter junctions of their associated transistors 33, 12 and 12' to protect them from excess turn off voltages. Similarly, a plurality of resistors 40, 42 and 44 are connected across the respective base emitter junctions of their associated transistors 34, 24 and 24' to provide protection from excess turn off voltages.

A pulsermeans embodied as a multivibrator stage 126 is used to provide a pulse output to operate the several output stages. Included in the multivibrator 126 are a pair of PNP transistors 134 and 136, which transistors are biased and coupled for alternate operation in the astable multivibrator mode. The transistors 134 and 136 are connected respectively through a pair of load resistors 138 and 140 to the negative terminal of a DC source 142. A pair of variable capacitors 144 and 146 are adjustable by ganged control and function to cross-couple the collectors of the transistors I34, 136, each to the opposing transistor base. A pair of signal limiting resistors 148, 150 and a potentiometer 152 are included in the multivibrator circuit with the on-off time of the multivibrator output controlled by the setting of the potentiometer 152. A pair of blocking diodes 154 and 156 are connected as shown in circuit with the movable contact of the potentiometer 152. The frequency of the multivibrator 126 operation and hence the frequency of the machining power pulses furnished to the machining gap or gaps may be selectively altered by changing the value of the capacitors 144, 146 through a ganged switch arrangement, such as is indicated by the dash lines. The pulse output from the multivibrator 126 is suitably amplified and resquared through a first common intermediate drive stage 31 and then through the separate following driver stages 30, 30a and 32, 32a already referred to above. The driver stages 30 and 30a are associated with the left hand machining gap, while the driver stages 32 and 32a are associated with the right hand machining gap.

While the present invention employs transistors as electronic switches, the invention is not so limited; but with proper redesign of the circuit by one skilled in the art any electronic switch may be substituted. By electronic switch I mean any electronic control device having three or more electrodes comprising at least two power electrodes acting to control curve flow in the power circuit, the conductivity of the power circuit being controlled by a control electrode within the switch, whereby the conductivity of the power circuit is regulated statically or electrically without movement of any mechanical elements within the switch. Included within this definition by way of illustration but not limitation are electronic tubes, transistors, silicon controlled rectifiers and the like.

Attention will now be given to the polarity control networks which are associated directly with the two machining gaps. A relay controlled network is shown associated with the diode of the left hand gap including the electrode 14 and the workpiece 16. A pair of relays are used to control the switching of two pairs of associated movable contacts to provide the several connections required to reverse the polarity to the two gap elements. At the left hand side of the gap, a relay R is shown, which relay controls a pair of jointly operable relay contacts indicated by the letters R, and R A second ralay S is connected at the right hand side of the gap which relay controls the joint movement of contacts S, and 8,. It will be seen that reponsive to the actuation of relay R through a suitable control switch the controls R, and R, will be moved leftwardly and a reverse polarity connection will be made in which the electrode 14 is maintained positive and the workpiece 16 negative. More specifically, this is accomplished by closing through contact R, the connection between the common lower end terminals of the resistors l8, l8 and the workpiece 16. At the same time, the closure of the contact R completes the circuit through the diode 20 from the positive terminal of the DC source 10 to provide a positive potential to the tool electrode 14. In the standard polarity condition, the relay S will be energized to place its normally opened contacts S, and S in their lower, closed position. Closureof contact S, will connect the lower terminals of resistors l8, 18' to the upper end of the electrode 14, thus permitting electron flow thereto from the negative terminal of the power source 10 through the collector emitter junctions of transistors 12 and 12. Closure of the right hand contact S will provide current flow through diode 20, through the contact S to the workpiece 16.

The polarity connections for the right hand gap are provided by a substantially identical switching network as that just described in connection with the left hand gap. Responsive to the energizing of the relay S, a pair of jointly operable contacts 8;, and S, will be closed to provide standard polarity connection. Responsive to the energization of the relay R, its associated contacts R and R, will be closed to provide the reverse polarity connection to the machining gap.

It will now be seen that there are available from the multiple machining gaps a pair of signals which indicate the magnitude of gap voltage. The gap signals will be maintained of a like polarity according to the selected energization of the relay R or of the relay S. To maintain the output signal to the servo system of a constant polarity independently of the relays selected, there are provided a pair of control networks, each of which network is associated with one individual gap. With respect to the left hand gap, the network includes a source of DC potential 169 and a pair of series resistors and 162 connected in series respectively with the emitter and the collector of a PNP transistor 164. A protective diode 166 is connected across the emitter base junction of the transistor 164. A resistor 167 is connected across its base collector junction. The elec- 'trode 14 of the machining gap has connected to it a parallel RC network, including a resistor 168 and a capacitor 170, to provide a normal minus signal from the output terminal labeled S when the setting for the machining gaps and for the polarity system is of the standard polarity. Altemately, where the polarity is of the reverse order, the movable contact, at the left side shown in its uppermost or S relay operated position, will be moved downwardly through the energization of the associated control relay R. While the relays denoted R are shown separately for convenience of illustration, it will be understood that relay control of the several contacts indicated and identified by the several R prefix designations may be operated through a single relay element. Likewise, control over the several contacts variously identified with the S prefix designations may be exercised either through one or through a plurality of relay elements.

A second control network is associated with the right hand machining gap, which network includes a source of potential 181 and a pair of resistors 172 and 174 connected respectively in series with the emitter and collector of a transistor 176. A protective diode 178 is connected across the emitter base junction, while a resistor 180 is connected across the collector base junction of the transistor 176 to protect it against excess turn off voltages. A signal input network, including a resistor 182 and a capacitor 184, is connected in series with the base of the transistor 176 for its turn on. Again, responsive to the energization of the relay R, the movement of the movable contact between the positions S and R may be controlled. We thus have two control outputs provided from the two network terminals labeled B.

In the FIG. 1 showing, the two outputs are connected to a utilization means which comprises a servo control circuit 185 shown at the lower left hand side of the drawing. A pair of like poled diodes 186 and 188 are connected as shown to a point A at the right hand terminal of the servo control network. Also connected in the input of the servo control network is a signal resistor 190. A differential amplifier is included in the servo circuit and includes a pair of PNP transistors 192 and 194. A DC source 196 has connected to its negative terminal a pair of load resistors 198 and 200, each associated with one of the two transistors 194, 192. The transistor 192 further has a protective diode 201 connected across its base emitter junction. The transistor 194 similarly has a protective diode 204 connected across its base emitter junction. The DC source 196 together with a potentiometer 206 provide a selectively adjustable reference voltage, which voltage is coupled through a signal resistor 208 to the base of the transistor 194. A capacitor 210 is connected between the movable contact of the potentiometer 206 and the positive terminal of DC source 196. A series resistor 216 is coupled between the positive terminal of the source 196 and the anodes of a pair of diodes 212 and 214. A series resistor 218 and a servo control coil 220 are coupled between the collectors of the transistors 192 and 194. It will be understood that servo control coil 220 is operatively connected to control the operation of a servo motor of the electrohydraulic type 'in a manner well known in the EDM art. The servo feed system may also be of the electrical motor driven type in which case one of the motor windings could be used as the electrical control element. An example of a complete electrohydraulic type servo system may be found in Robert S. Webb U. S. Pat. No. 3,230,412 entitled SERVO FEED APPARATUS FOR ELECTRI- CAL DISCHARGE MACHINING, which patent is of common ownership herewith.

It will be seen that a preset reference voltage is connected to the control electrode or base of the transistor 194, while a gap control voltage signal which is of variable magnitude but always has the same polarity, is presented to the control electrode or base of the right hand transistor 192. Responsive to a significant drop of gap voltage at either of the two gaps being sensed, the signal at the base of the transistor 192 will drop. During a gap short circuit condition, the transistor 194 will be turned on, while the transistor 192 will be turned off. Accordingly, the direction of electron flow will be through the resistor 198, through the collector emitter junction of the transistor 194, through the diode 212 and the resistor 216 to the positive terminal of the DC source 196. At the same time, there will be an electron flow from the negative terminal of the DC source 196, through the resistor 200, through the resistor 218, and leftwardly through the servo control coil 220 to provide a continuing up-feed direction of servo operation for the duration of the gap short circuit condition.

During normal gap voltage level, the phasing of the transistors I92, 194 will be the opposite one. Transistor 192 will be turned on and transistor 194 off. This will result in electron flow through the resistor 200, through the collector emitter junction of the transistor 192. through the diode 214 and the resistor 216 to the plus terminal of the DC source 196. At the same time, there will occur an electron flow from the negative terminal of the DC source 196 through the resistor 198, rightwardly in the down-feed direction through the servo coil 220 and through the resistor 218 and then through the transistor 192 and in the path previously described.

It will thus be seen that I have through the embodiments shown in FIG. 1 provided a constant control output for the servo system without respect to the selected polarity of the gaps.

FIG. 2 shows a system identical to FIG. 1 so far as the polarity reversing relays and the polarity control networks for each gap are concerned. In this case, however, the constant polarity output signals from the two control networks are used to control separate cut-off circuits associated with the output stages for the two different gaps. With respect to the right hand gap including the electrode 26 and the workpiece 28, there is provided a cut-off circuit 25 which is shown with two signal leads, both coupled to the several input stagesfor the transistors 24, 24. The right hand lead 27 provides to the cut-off circuit 25 a signal representative of gap on-time which provides a keying signal for the cut-off circuit. The left hand lead 29, which is shown with its upper end connected between the drive stages 32 and 32a, is used to provide a control cut-off signal for the drive. Essentially, the cut-off operation is achieved by shunting drive signal from the output electronic switch network. A-second cut-off circuit 25 is shown in full detail at theleft hand side of the drawing. The components and mode of operation of the cut-off circuits 25 and 25' are identical. Included in the cut-off circuit 25' is the input received from the polarity control network. A keying signal lead is connected through a pair of limiting resistors 250, 252 to the output of driver stage 300'. Included in the first stage of cut-off circuit 25' is a transistor 253, which transistor has its base in series with a drive signal resistor 254 and a base resistor 256. The bias resistor 256 has its right hand terminal connected to the negative terminal of a DC source 258. The transistor 253 further has its base emitter junction shunted by diode 260 and a parallel capacitor 262. The keying signal input, which is of a minus polarity, is passed through a diode 264 to the emitter of the transistor 253. A reference voltage is coupled to the cathode of the diode 264, whichreference voltage includes a DC supply 266, a shunt potentiometer 268 and a capacitor 270 connected between the positiveterminal of the bias source 266 and the movable contact of the potentiometer 268. A diode 272 is connected between the keying signal input and the reference voltage. Connected in series with the collector of the transistor 253 is a load resistor 274. A transistor stage next following includes a PN P transistor 276 which has a protective diode 278 across its base emitter junction and a current limiting resistor 280 connected in series with its base. A collector lead resistor 282 is further connected between the transistor 276 collector and the negative terminal of DC source 258. The next stage in the cut-off circuit includes a pair of alternately conductive transistors 284 and 286. The transistor 284 is connected to the output of the transistor 276 through a signal limiting resistor 288 with a bias furnished to its base through a resistor 290. The collector of the transistor 284 is connected to a load resistor 292 and it further has a protective diode 244 connected across its base emitter junction. A second diode 296 poled in the manner shown is connected in series with the emitter of the transistor 284. The transistor 286 has connected to its collector a load resistor 298, with a bias resistor 300 connected between its base and the positive terminal of the DC source 258. The circuit is completed by a pair of RC networks, including a shunt network with resistor 302 and capacitor 304 and the shunt network including resistor 306 and capacitor 308. A resistor 311 is connected in circuit with the two transistors 284 and 286 as shown.

The control output from cut-off circuit 25 is provided from the collector of a final cut-off transistor stage including a transistor 312, which transistor has its base connected to the left hand terminal of the RC shunt network including resistor. 306 and capacitor 308. Responsive to the conduction of the transistor 312, the drive signal is shunted away from the next following stage 30a through a diode 316. During a gap short circuit condition, the following is the sequence of operation of the several stages of the cut-off circuit: the transistors 253 and 276 are turned on, the transistor 284 is switched off, and the transistor 312 is finally turned on to interrupt drive signal.

The operation of the cut-off circuit 25 or 25' thus proceeds independently of whether the gap polarity is standard or reversed. The mode of operation possible now makes it feasible to provide a single pulser means, such as the multivibrator 126, to operate a number of drive stages and electronic switch output stages to provide cutting over a plurality of machining gaps at the same time. The cut-off circuit is operable beyond the.

first common drive stage of the machining gap circuits so that if one of the several gaps is in a shorted condition, it may temporarily be interrupted while machining is permitted to progress uninterrupted at the other gaps. This arrangement provides much greater flexibility and permits ready change-over of the power supply between different types of EDM operation. Of particular importance to the'embodiments of both FIGS. 1 and 2 is-the arrangement for polarity reversal and the control network for providing a constant polarity control signal from one gap or from the several gaps independently of the preselected'gap polarity.

I claim:

1. In an electrical discharge machining apparatus for machining a conductive workpiece gap element by electrical discharges passed from a tool electrode gap element across a dielectric coolant filled gap, a periodically operated electronic output switch and a DC power source operatively connected across said gap for providing machining power pulses thereto, a system for selectively reversing the relative polarities of said gap elements, said system comprising a first switching means operatively connected between said output switch and one of said gap elements, a second switching means operatively connected between said output switch and the other of said elements, both of said switching means selectively actuable for connecting said output switch to a respective one of said gap elements and, at the same time, connecting a like terminal of said source to the other, and a control network operatively connected to one of said gap elements for providing a constant polarity control signal to a utilization means in said apparatus, said network including a third switching means operable in unison with said two first mentioned switching means to provide a first signal path direct from said gap to said utilization means, and

an inverter stage, said third switching means operable to provide a second signal path from said gap through said inverter stage to said utilization means. i

2. In an electrical discharge machining apparatus for machining a conductive workpiece gap element by electrical discharges passed from a tool electrode gap element across a dielectric coolant filled gap, a periodi cally operated electronic output switch and a DC power source operatively connected to said gap for providing machining power pulses thereto, wherein the improvement comprises a system for selectively reversing the relative polarities of said gap elements, said system comprising a first switching means operatively connected between said output switch and one of said gap elements and a second switching means operatively connected between said output switch and the other of said gap elements, both of said switching means selectively actuable for connecting said output switch to a respective one of said gap elements and, at the same time, connecting a like terminal of said source to the other, a servo feed means operably connected to one of said gap elements to provide feed movement relative to the other, and a control network coupled to the output of said gap for providing a constant polarity control signal to said servo feed means, said network including a third switching means actuable in unison with said first mentioned two switching means to provide a first signal path direct from said gap to said servo means and a second signal path from said gap through an inverter stage to said servo means.

3. The combination as set forth in claim 2 wherein said servo means includes a differential amplifier wherein a reference voltage source is connected to one input of said amplifier and said constant polarity control signal is connected to the other input of said differential amplifier.

4. The combination as set forth in claim 3 wherein said differential amplifier includes a pair of alternately operable transistors and wherein an electrical control element for said servo means is connected between like power electrodes of said transistors for operation.

5. The combination as set forth in claim 2 wherein said inverter stage includes a transistor having its control electrode coupled to said gap through a sensing network and one of its principal electrodes operably coupled to said servo means. 1 g

6. In an electrical discharge machining apparatus for machining a conductive workpiece gap element by electrical discharges passed from a tool electrode gap element across a dielectric coolant filled gap, a periodically operated electronic output switch and a DC power source operatively connected to said gap for providing machining power pulses thereto, a system for selectively reversing the relative polarities of said gap elements, said system comprising a first switching means operatively connected between the output of said output switch and one of said gap elements and a second switching means operatively connected between said output switch and the other of said elements, said switching means each selectively actuable for connecting said output switch to a respective one of said gap elements and, at the same time connecting a like terminal of said source to the other, a protective cutoff means operably connected to said output switch. to interrupt machining power pulses to the gap responsive to gap-short circuit condition, and a control network coupled to the output of said gap for providing a constant polarity control signal to said cut-off means, said network including a third switching means actuable in unison with said first mentioned pair of switching means to provide a first signal path direct from said gap to said cut-off means and a second signal path from said gap through an inverter stage to said cut-off means.

7. The combination as set forth in claim 6 wherein said inverter stage comprises a transistor having its control electrode coupled to said gap through a sensing network and one of its principal electrodes operatively connected to said cut-off means.

8. The combination as set forth in claim 6 wherein a keying signal lead is operatively connected to said cutoff means for rendering it operable during each of said machining power pulses.

9. The combination as set forth in claim 8 wherein said cut-off means includes a cut-off electronic switch operatively connected to a drive signal source for said electronic output switch for shunting its drive signal responsive to gap short circuit condition.

10. The combination as set forth in claim 9 wherein a pulsing means and a plurality of intermediate drive stages are connected to said periodically operated electronic output switch for controlling its conduction, and wherein said keying signal lead is operatively connected to one of said drive stages and wherein an output from a control electrode of said cut-off electronic switch is coupled to shunt drive signal from the other of said drive stages.

11. In an electrical discharge machining apparatus for machining at least one conductive workpiece by electrical discharges passed from a plurality of spaced toolelectrodes across a like plurality of dielectric coolant filled gaps, a periodically operated electronic output switch and a separate DC power source operatively connected to each of said gaps for providing machining power pulses thereto, wherein the improvement comprises a system for selectively reversing the relative polarity of said tool electrodes and said workpiece, said system comprising for each gap a first group of switching means, each operatively connected between the output of each of said switches and a related one of said electrodes and a second group of switching means, each operatively connected between the output of each of said switches and said workpiece, said first group of switching means selectively actuable for connecting each of said output switches to their respective tool electrode and, at the same time, connecting like terminals of said sources to said workpiece, said second group of switching means selectively actuable for connecting each of said output switches to said workpiece and at the same time connecting like terminals of said source each to a different one of said tool electrodes, a common servo feed means included to provide simultaneous feed of all of said electrodes relative to said workpiece and a control network coupled to the output of each of said gaps for providing a constant polarity control signal to said servo means, said control network including a third switching means operable in unison with said first mentioned two groups of switching means to provide a first signal path direct from said gap to said servo means and a second signal path from said gap through an inverter stage to said servo means.

12. The combination as set forth in claim 1 1 wherein said inverter stage includes an electronic switch having its control electrode coupled to said associated gap through a shunt capacitor-resistor sensing network and one of its principal electrodes coupled to the output of said network.

13. The combination as set forth in claim 12 wherein said servo means includes a differential amplifier comprising two alternately conductive electronic switches, wherein said plurality of networks are each coupled to the control electrode of one of said electronic switches through like poled diodes, and wherein a selectively settable reference voltage is connected to the control electrode of the other of said electronic switches of said differential amplifier.

14. The combination as set forth in claim 13 wherein said servo means is of the electrohydraulic type and wherein a control coil for said servo means is connected intermediate corresponding principal electrodes of said electronic switches of said differential amplifier.

15. In an electrical discharge machining apparatus for machining at least one conductive workpiece by electrical discharges passed from a plurality of spaced tool electrodes across a like plurality of dielectric coolant filled gaps, a periodically operated electronic output switch and a separate DC power source operatively connected to each of said gaps for providing machining power pulses thereto, a system for selectively reversing the relative polarity of said electrodes and of said workpiece, said system comprising for each gap a separate first switching means operatively connected between the output of each of said switches and a different one of said electrodes, a second switching means operatively connected between the output of each of said switches and said workpiece, all of said first switching means selectively actuable for connecting each of said output switches to a respective tool electrode and, at the same time, connecting a like terminal of said sources to said workpiece, all said second switching means selectively actuable for connecting each of said output switches to said workpiece and, at the same time, connecting like terminals of said sources each to a different one of said tool electrodes, a separate cutoff means operatively connected to and controlling the drive signal for each of said electronic output switches,

and a control network coupled to the output of each of said gaps for providing a constant polarity control signal to its cut-off means, said control network including in each instance a third switching means operable in unison with said first mentioned pair of switching means to provide a first signal path direct from said gap to said cut-off means and a second signal path from said gap through an inverter stage to said cut-ofi means.

16. The combination as set forth in claim 15 wherein a common pulser means is included and a separate drive stage is included between said pulser means and each of said periodically operated electronic output switches, and wherein a keying signal is provided to said cut-off means for rendering it operable in phase with said machining power pulses. 4

17. The combination as set forth in claim 16 wherein said cut-off means includes a control output coupled to a respective one of said drive stages for shunting drive signal from said electronicoutput switch responsive to gap short circuit condition.

said first and second switching means comprise electromagnetic relays and wherein a common operator means is included for said relays to provide like polarity connection for all of said gaps at the same time. 

1. In an electrical discharge machining apparatus for machining a conductive workpiece gap element by electrical discharges passed from a tool electrode gap element across a dielectric coolant filled gap, a periodically operated electronic output switch and a DC power source operatively connected across said gap for providing machining power pulses thereto, a system for selectively reversing the relative polarities of said gap elements, said system comprising a first switching means operatively connected between said output switch and one of said gap elements, a second switching means operatively connected between said output switch and the other of said elements, both of said switching means selectively actuable for connecting said output switch to a respective one of said gap elements and, at the same time, connecting a like terminal of said source to the other, and a control network operatively connected to one of said gap elements for providing a constant polarity control signal to a utilization means in said apparatus, said network including a third switching means operable in unison with said two first mentioned switching means to provide a first signal path direct from said gap to said utilization means, and an inverter stage, said third switching means operable to provide a second signal path from said gap through said inverter stage to said utilization means.
 2. In an electrical discharge machining apparatus for machining a conductive workpiece gap element by electrical discharges passed from a tool electrode gap element across a dielectric coolant filled gap, a periodically operated electronic output switch and a DC power source operatively connected to said gap for providing machining power pulses thereto, wherein the improvement comprises a system for selectively reversing the relative polarities of said gap elements, said system comprising a first switching means operatively connected between said output switch and one of said gap elements and a second switching means operatively connected between said output switch and the other of said gap elements, both of said switching means selectively actuable for connecting said output switch to a respective one of said gap elements and, at the same time, connecting a like terminal of said source to the other, a servo feed means operably connected to one of said gap elements to provide feed movement relative to the other, and a control network coupled to the output of said gap for providing a constant polarity control signal to said servo feed means, said network including a third switching means actuable in unison with said first mentioned two switching means to provide a first signal path direct from said gap to said servo means and a second signal path from said gap through an inverter stage to said servo means.
 3. The combination as set forth in claim 2 wherein said servo means includes a differential amplifier wherein a reference voltage source is connected to one input of said amplifier and said constant polarity control signal is connected to the other input of said differential amplifier.
 4. The combination as set forth in claim 3 wherein said differential amplifier includes a pair of alternately operable transistors and wherein an electrical control element for said servo means is connected between like power electrodes of said transistors for operation.
 5. The combination as set forth in claim 2 wherein said inverter stage includes a transistor having its control electrode coupled to said gap through a sensing network and one of its principal electrodes operably coupled to said servo means.
 6. In an electrical discharge machining apparatus for machining a conductive workpiece gap element by electrical dIscharges passed from a tool electrode gap element across a dielectric coolant filled gap, a periodically operated electronic output switch and a DC power source operatively connected to said gap for providing machining power pulses thereto, a system for selectively reversing the relative polarities of said gap elements, said system comprising a first switching means operatively connected between the output of said output switch and one of said gap elements and a second switching means operatively connected between said output switch and the other of said elements, said switching means each selectively actuable for connecting said output switch to a respective one of said gap elements and, at the same time connecting a like terminal of said source to the other, a protective cut-off means operably connected to said output switch to interrupt machining power pulses to the gap responsive to gap short circuit condition, and a control network coupled to the output of said gap for providing a constant polarity control signal to said cut-off means, said network including a third switching means actuable in unison with said first mentioned pair of switching means to provide a first signal path direct from said gap to said cut-off means and a second signal path from said gap through an inverter stage to said cut-off means.
 7. The combination as set forth in claim 6 wherein said inverter stage comprises a transistor having its control electrode coupled to said gap through a sensing network and one of its principal electrodes operatively connected to said cut-off means.
 8. The combination as set forth in claim 6 wherein a keying signal lead is operatively connected to said cut-off means for rendering it operable during each of said machining power pulses.
 9. The combination as set forth in claim 8 wherein said cut-off means includes a cut-off electronic switch operatively connected to a drive signal source for said electronic output switch for shunting its drive signal responsive to gap short circuit condition.
 10. The combination as set forth in claim 9 wherein a pulsing means and a plurality of intermediate drive stages are connected to said periodically operated electronic output switch for controlling its conduction, and wherein said keying signal lead is operatively connected to one of said drive stages and wherein an output from a control electrode of said cut-off electronic switch is coupled to shunt drive signal from the other of said drive stages.
 11. In an electrical discharge machining apparatus for machining at least one conductive workpiece by electrical discharges passed from a plurality of spaced tool electrodes across a like plurality of dielectric coolant filled gaps, a periodically operated electronic output switch and a separate DC power source operatively connected to each of said gaps for providing machining power pulses thereto, wherein the improvement comprises a system for selectively reversing the relative polarity of said tool electrodes and said workpiece, said system comprising for each gap a first group of switching means, each operatively connected between the output of each of said switches and a related one of said electrodes and a second group of switching means, each operatively connected between the output of each of said switches and said workpiece, said first group of switching means selectively actuable for connecting each of said output switches to their respective tool electrode and, at the same time, connecting like terminals of said sources to said workpiece, said second group of switching means selectively actuable for connecting each of said output switches to said workpiece and at the same time connecting like terminals of said source each to a different one of said tool electrodes, a common servo feed means included to provide simultaneous feed of all of said electrodes relative to said workpiece and a control network coupled to the output of each of said gaps for providing a constant polarity control signal to said servo means, said control network including a third switching means operable in unison with said first mentioned two groups of switching means to provide a first signal path direct from said gap to said servo means and a second signal path from said gap through an inverter stage to said servo means.
 12. The combination as set forth in claim 11 wherein said inverter stage includes an electronic switch having its control electrode coupled to said associated gap through a shunt capacitor-resistor sensing network and one of its principal electrodes coupled to the output of said network.
 13. The combination as set forth in claim 12 wherein said servo means includes a differential amplifier comprising two alternately conductive electronic switches, wherein said plurality of networks are each coupled to the control electrode of one of said electronic switches through like poled diodes, and wherein a selectively settable reference voltage is connected to the control electrode of the other of said electronic switches of said differential amplifier.
 14. The combination as set forth in claim 13 wherein said servo means is of the electrohydraulic type and wherein a control coil for said servo means is connected intermediate corresponding principal electrodes of said electronic switches of said differential amplifier.
 15. In an electrical discharge machining apparatus for machining at least one conductive workpiece by electrical discharges passed from a plurality of spaced tool electrodes across a like plurality of dielectric coolant filled gaps, a periodically operated electronic output switch and a separate DC power source operatively connected to each of said gaps for providing machining power pulses thereto, a system for selectively reversing the relative polarity of said electrodes and of said workpiece, said system comprising for each gap a separate first switching means operatively connected between the output of each of said switches and a different one of said electrodes, a second switching means operatively connected between the output of each of said switches and said workpiece, all of said first switching means selectively actuable for connecting each of said output switches to a respective tool electrode and, at the same time, connecting a like terminal of said sources to said workpiece, all said second switching means selectively actuable for connecting each of said output switches to said workpiece and, at the same time, connecting like terminals of said sources each to a different one of said tool electrodes, a separate cut-off means operatively connected to and controlling the drive signal for each of said electronic output switches, and a control network coupled to the output of each of said gaps for providing a constant polarity control signal to its cut-off means, said control network including in each instance a third switching means operable in unison with said first mentioned pair of switching means to provide a first signal path direct from said gap to said cut-off means and a second signal path from said gap through an inverter stage to said cut-off means.
 16. The combination as set forth in claim 15 wherein a common pulser means is included and a separate drive stage is included between said pulser means and each of said periodically operated electronic output switches, and wherein a keying signal is provided to said cut-off means for rendering it operable in phase with said machining power pulses.
 17. The combination as set forth in claim 16 wherein said cut-off means includes a control output coupled to a respective one of said drive stages for shunting drive signal from said electronic output switch responsive to gap short circuit condition.
 18. The combination as set forth in claim 15 wherein said inverter stage includes an electronic switch having its control electrode operably coupled to said gap through a sensing network and one of its principal electrodes coupled to the output of said network.
 19. The combination as set forth in claim 15 whErein said first and second switching means comprise electromagnetic relays and wherein a common operator means is included for said relays to provide like polarity connection for all of said gaps at the same time. 